Wikipedia arsenic -
co-occurrence of ores led to copper arsenic bronzes but fell out of favour, danger and hit and miss results depending on ore.
Journal of Archaeological Science, Volume 26, Number 5, May 1999, pp. 497-526(30)
copper oxide with copper or iron sulpharsenide 2:1 to 4:1 gives clean metal not flecked by slag
see also lechtman
wikipedia smelting -
Bronze is a copper/arsenic or copper/tin alloy. The presence of arsenic and tin dramatically increased the hardness of copper, producing war-winning weapons and armor. A noble wearing bronze armor was basically impervious to the stone tools of the times, and his bronze sword kept its edge and shattered the older stone-based weapons. The knowledge of the smelting of copper allows kings to overcome their enemies, and caused such a revolution that it marked the end of the Stone Age and the beginning of the Bronze Age. It would be millenia, though, until bronze could be used by common soldiers and townsfolk, and for a long time they were luxury items used by nobility.
The first copper/arsenic bronzes date of 4200BC from Asia Minor, and were used for a long time until replaced by the modern copper/tin bronzes by 1500BC. It is unclear that if at some point in time the smiths that produced copper/arsenic bronze added arsenic oxides on purpose, or if they explored some copper lodes that happened to have arsenic as a lucky contamination.
The first copper/tin bronzes date of 3200BC, again from Asia Minor. Copper/tin bronzes are harder and more durable than copper/arsenic ones, and made these obsolete. The process through which the smiths learned to produce copper/tin bronzes is once again a mystery. The first such bronzes were probably a lucky accident from tin contamination of copper ores, but by 2000BC we know that tin was being mined on purpose for the production of bronze. This is amazing, given that tin is a semi-rare metal, and even a rich cassiterite ore only has 5% tin. Also, cassiterite looks like any common rock, and it takes special skills (or special instruments) to find it and locate the richer lodes. But, whatever steps were taken to learn about tin, these were fully understood by 2000BC.